A. Field of Invention
This invention pertains to implantable cardiac devices (ICDs) such as pacemakers, and more particularly to such devices with means for distinguishing ambient electronic noise from arrhythmic cardiac behavior such as abnormally fast heart beats so as to provide appropriate and reliable therapy to a patient even in the presence of low level continuous noise, caused for example by electrical interference.
B. Description of the Prior Art
Typically ICDs monitor a patient's heart condition by sensing the Intra Cardiac Electrogram (IECG), a voltage that occurs when the cardiac muscle depolarizes at the beginning of each heart contraction. The IECG is sensed by one or two electrodes placed in or near the heart, where it appears as a voltage waveform which is normally referred to as the QRS complex. The IECG QRS complex during normal heart beats has an amplitude of about 15 to 25 mV and a broad frequency spectrum in the range of about 40 to 70 Hz. There are numerous systems in the prior art that detect the QRS complex by amplifying the IECG signal, filtering it to reduce noise and then detecting the QRS complex with an amplitude threshold detector. The threshold detector may have a fixed threshold or adapt the threshold to the changing amplitude of the signal.
A problem with this method of detecting cardiac activity is that because of its low amplitude and particular spectral characteristics, the IECG can be corrupted by ambient signals such as noise or artefact from natural and artificial sources. Common sources of such ambient signals include electromyographic noise from skeletal muscles near the electrodes, contact noise from intermittent contact in the electrode circuit (either the electrode wiring or between the electrode and heart muscle as the heart moves) and radiated or induced voltages from voltage lines and other external power sources. These ambient signals appear as waveforms with an amplitude and frequency content similar to the QRS complex and thus may confuse the sensing system of the ICD and cause incorrect therapy to be applied to the patient. This problem is compounded by the nature of the IECG of some arrhythmias, in particular, ventricular fibrillation, which has a lower amplitude (0.2 to 5 mv), higher rate (300 to 400 beats/min.) and lower frequency content than the normal IECG.
A common method used in many pacemakers to resolve this problem is referred to as the noise sensing window. This method takes advantage of the fact that even the fastest natural heart rates result in signals which follow a QRS complex by at least 300-400 ms. Accordingly, a noise window is designated as a window typically 100-120 ms after a QRS complex is sensed. Any signals sensed in this window are assumed to be noise.
A variation of this method is described in U.S. Pat. No. 4,173,230 to Digby entitled "Noise Elimination and Refractory Period Control In Demand Pacemakers" where a combination of filters and refractory periods are used to reduce the effect of noise.
Another noise detection method is described in U.S. Pat. No. 4,694,931 to Beck entitled "Sampled Data Sense Amplifier." The device disclosed therein searches for discontinuities in the background level of the sensed signal, and therefore ignores any noise if it is continuous and uniform.
U.S. Pat. No. 5,395,393 discloses a noise elimination method using variable amplifier thresholds.
These prior art noise detection systems have inherent problems of their own. One such problem exists if the noise level is near the detection threshold, or is of variable amplitude. In this situation, the detection is intermittent, thus providing a variable classification of noise that may lead to intermittent or indeterminate delivery of therapy to the patient. Additionally, these systems fail to work effectively when used for arrhythmia detection where an arrhythmia maybe of sufficiently high rate to cause a valid detection to fall in the noise window.
The problem of these systems misclassifying ventricular fibrillation (VF) as noise is significant. This is because the IECG for VF is highly variable and can have short intervals that have very similar characteristics to sine waves induced of by conventional power lines. This is of major consequence to patients with ICDs which require fast and accurate diagnosis of VF for the ICD to effectively save their lives from this lethal arrhythmia.
Other schemes among automatic sensing threshold, such as for example in my co-pending U.S. application Ser. No. 700,730 filed Aug. 13, 1996, entitled APPARATUS AND METHOD OF NOISE CLASSIFICATION IN AN IMPLANTABLE CARDIAC DEVICE, now U.S. Pat. No. 5,702,495. However, these schemes are very sensitive to noise at low heart rate. Moreover, these schemes are also very sensitive to noise during the asystole.
Some implantable cardiac devices are shown in U.S. Pat. No. 5,048,521 and 5,111,816.